Myocardial regeneration after infarction mediated by injection of autologous cardiac progenitor cells (CPCs) or local activation of resident CPCs by growth factors results in a significant recovery of ventricular muscle mass. However, the regenerated myocytes are small and have fetal-neonatal characteristics. These cells tend not to increase in size over time and in the majority of cases fail to attain the properties of adult fully mature myocytes. In contrast, the occasional migration of CPCs from the border zone to the remote myocardium results in the formation of myocytes which are indistinguishable from preexisting adjacent cells. This difference in CPC behavior suggests that the differentiation of CPCs may depend on surrounding cells which are connected through gap junctions to the primitive cells. MicroRNAs (miRs) which are emerging as an unexpected mechanism of control of progenitor cell fate are small RNAs that have the ability to traverse gap junctions. In this application I am raising the hypothesis that miR-499 that is highly expressed in cardiomyocytes and minimally present or absent in undifferentiated human CPCs may be involved in the maintenance of the terminally differentiated state of myocytes and may translocate to neighboring CPCs initiating their differentiation. Preliminary data demonstrate that miR-499 translocate from donor cells to neighboring human CPCs through gap junctions and the accumulation of miR-499 in CPCs is coupled with a decreased expression of the target genes, Sox6 and Rod1. The possibility is advanced that SoxD proteins, Sox5 and Sox6, negatively modulates the function of the members of the SoxF group of proteins inhibiting cardiomyogenesis. Sox5 and Sox6 may oppose the activation of the Wnt-2-catenin pathway which may condition late differentiation of human CPCs into cardiomyocytes. Based on this hypothesis, in vivo studies will be conducted to demonstrate whether overexpression of miR-499 in human CPCs promotes myocyte differentiation in the infarcted heart. PUBLIC HEALTH RELEVANCE: One of the major problems in cardiac repair is the lack of maturation of the newly formed cardiomyocytes. The work proposed here may overcome this limitation and offer a novel strategy for the treatment of the infarcted heart. If successful, this work may have important clinical implications for the management of human heart failure.